The present invention is directed, in general, to semiconductor devices and, more specifically, to a bipolar transistor having an emitter comprised of a semi-insulating material and a method of manufacturing the same.
Bipolar technology has been extensively used through the years for applications requiring high speed, high current drive, and low noise. Scaling of semiconductor devices has not only drastically increased the density of integrated circuits, but it has also improved device and circuit performance. This rapid advance in semiconductor technology has placed increasing pressure on the ability of bipolar devices to dominate high-performance applications. Among the primary ways bipolar devices, such as bipolar transistors, may be improved for high-performance applications is to reduce the overall capacitance of the device, for example, between the base and emitter, to arrive at a faster switching device or a higher cutoff frequency. Those skilled in the art understand that decreasing device capacitance can result in an increase in device speed. However, the typically high dopant concentrations implanted into these layers of the device often increase capacitance across the device, for example, across the base and emitter.
Unfortunately, conventional techniques used to decrease this component of device capacitance, and thereby increase device switching speed, are at odds with the additional desire to increase or maintain the current gain of such bipolar devices. More specifically, by reducing the dopant concentrations to be implanted in the device layers, the amount of current gain of a heterojunction bipolar device may also be reduced. As such, reducing the amount of current gain in a heterojunction bipolar device by reduction in emitter doping results in a reduction in device capacitance, and thus an increase in switching speed or cutoff frequency.
As is well known in the art, current gain is exponentially related to the xe2x80x9cbandgapxe2x80x9d differential of the materials used to form the base and emitter layers in a heterojunction bipolar device. The bandgap for a given material may be broadly described as the minimum energy required to excite electrons within that material sufficient to transfer electrons from the valence band to the conduction band, such that they may contribute to electrical conduction. Typically, insulators have an extremely large bandgap, semi-insulating films have a relatively large bandgap, semiconductors have an average bandgap, and metal conductors have little or no bandgap at all. In a heterojunction device, the larger the differential between the bandgap of a material comprising the base and the bandgap of a material comprising the emitter of a bipolar device, the larger the current gain across the device.
Conventional techniques aimed at increasing overall switching speed for bipolar devices employ this bandgap-current gain relationship. In order to reduce capacitance across the device and typically also allowing the use of reduced dopant concentration levels in the device layers, semiconductor manufacturers attempt to xe2x80x9cexchangexe2x80x9d some current gain for the reduction in capacitance. More specifically, the bandgap differential between the emitter or base of a bipolar device is increased as much as possible so that some of the current gain achieved may be exchanged for reduced capacitance across the base and emitter. As a result, although not all of the current gain originally achieved is maintained, a portion of it is retained and a portion of it is exchanged for reduced device capacitance and increased device speed by permitting reduced dopant concentration levels in the device layers.
With heterojunction bipolar devices, an increase in bandgap differential in present technology is typically sought by decreasing the bandgap of the base layer. However, recent approaches to increasing switching speed have included increasing the bandgap differential across a typical device by increasing the bandgap of the emitter layer, rather than the bandgap of the base, thus converting the device to a heterojunction bipolar device. However, even these attempts have not increased the bandgap differential between the base and emitter of a bipolar device to arrive at the switching speeds demanded by today""s, as well as tomorrow""s, semiconductor market.
Many of the conventional techniques attempting to maintain a high current gain while simultaneously increasing device performance have produced relatively disappointing results. Early attempts to achieve current gains high enough to xe2x80x9cexchangexe2x80x9d for reduced capacitance have employed tunneling insulators sandwiched in between the emitters and bases of bipolar devices. For example, techniques employing silicon dioxide (SiO2) with a polysilicon cap as an emitter material have been successful in achieving barrier heights for holes on the order of 1.0 eV. This increases the gain of the device, however, the dopant concentrations in the polysilicon required to make the emitter resistance adequately low in a bipolar device is about 1E16/cm2. The result is remaining excessive parasitic capacitance across the base and emitter of the device.
Accordingly, what is needed in the art is a bipolar device, and related method of manufacturing, which maintains a high overall current gain with relatively low base-to-emitter capacitance that does not suffer from the deficiencies found in the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a bipolar transistor. In one embodiment, the bipolar transistor includes a collector located in a semiconductor substrate having a given bandgap, and a base in contact with the collector. The base preferably has a bandgap less than the bandgap of the substrate. In addition, the bipolar transistor further includes an emitter located over the base, where the emitter has a bandgap greater than the bandgap of the substrate.
In another aspect of the present invention, a method of manufacturing a bipolar transistor is disclosed. In one particular embodiment, the method includes depositing a collector in a semiconductor substrate having a given bandgap, and creating a base in contact with the collector. The base preferably has a bandgap less than the bandgap of the substrate. The method further includes forming an emitter having a bandgap greater than the bandgap of the substrate over the base.
In yet another aspect of the present invention, an integrated circuit is disclosed. In one embodiment, the integrated circuit includes passive devices formed over a semiconductor substrate having a given bandgap, as well as interlevel dielectric layers. In addition, the integrated circuit includes a bipolar transistor having a collector located in the substrate and a base in contact with the collector. The base has a bandgap less than the bandgap of the substrate. The bipolar device further includes an emitter, having a bandgap greater than the bandgap of the substrate, located over the base. The integrated circuit further includes interconnections, created in the dielectric layers, connecting the passive devices and the bipolar transistor to form an operative integrated circuit.
The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.